Chemistry Reference
In-Depth Information
The radical concentration during the polymerization is not changed by reaction
(8-49), but the initiator molecule involved has been wasted because its decompo-
sition has not produced a net increase in the conversion of monomer to polymer.
Induced decomposition reactions are negligible for azo initiators, but they can be
very significant for some peroxides. Peroxydicarbonates, for example, have effi-
ciencies that change greatly with reaction conditions.
The major cause of primary radical wastage involves “cage” reactions. When
an initiator decomposes, the primary radicals are each other's nearest neighbors
for about 10
2
10
sec. During this interval they are surrounded by a “cage” of sol-
vent and monomer molecules through which they must diffuse to escape each
other. Once one or the other radical leaves the cage it is extremely unlikely that
the pair will encounter each other again. While they are in the cage, however, any
reaction that takes place will be between the two primary radicals which can be
expected to be colliding on the average once every 10
2
13
sec. (The vibrational
frequency of a diatomic molecule at reaction temperatures is approximately
10
13
sec
2
1
.) Direct recombination simply regenerates the original initiator mole-
cule, but other reactions can also occur that waste the radicals for polymerization.
To illustrate, the decomposition of acetyl peroxide could lead to the following
events:
[
CH
3
+
]
CH
3
COO
C
CH
3
C
CH
3
C
O
O
O
O
(8-50)
]
[2CH
3
C
2CH
3
C
(8-51)
O
O
CH
3
CH
3
C
+CO
2
(8-52)
O
]
[2CH
3
C
[2CH
3
COCH
3
+
CO
2
]
(8-53)
O
O
]
[2CH
3
C
[CH
3
CH
3
+
2CO
2
]
(8-54)
O
Here the brackets indicate caged radicals, and the last two reactions waste the
initiator. Only reactions (8-51) and (8-52) yield radicals that could initiate poly-
merization. All initiators suffer cage wastage reactions. At 60
C, for example,
f
for AIBN is only 0.6 because of these events.
